Drilling platforms

In 2007, 982 oil and gas platforms stood in the North Sea. In addition to the drilling rigs, this figure includes service rigs and platforms no longer in use but not yet dismantled. Great Britain and Norway own the majority of the oil and gas platforms, namely 590 and 193 respectively. The Netherlands 143 platforms in the North Sea, Denmark 53 and Germany 3. Almost all of the platforms in the southern North Sea are for gas exploitation. Both oil and gas installations are located in the central and northern part of the North Sea. There are installations on the production platforms which exploit oil or gas from the sea floor. Sometimes there are living accommodations for the workers on the rig, however they can also be unmanned. Other platforms are specially designed for trans-shipment of the oil or for the handeling of the gas so that it can be transported to the mainland via the pipeline. There are special platforms for trial drills.

Between 1980 and 2006, the number of production platforms in the Dutch section of the continental plate (NCP) grew from 20 to 156. Every year, depending upon the demand and the license policy, between four to thirty trial drills for minerals are made on the NCP. In total, more than 2500 kilometers of pipelines for the benefit of mineral exploitation lie on or in the bottom of the NCP.

Types of drilling platforms

In the North Sea, the mining companies use two basic types of production platforms. Both have a substructure and a superstructure. Some substructures are built from steel while others are constructed from cconcrete.

The drilling rigs with steel substructures are built totally on shore. The completed construction is then towed to the oil or gas field. Upon arrival, the substructure is anchored in the bottom, sometimes down to 100 meters. Afterwards, floating cranes install the superstructure which has been towed separately to the location. A steel production platform does not have storage capacity and is therefore dependent upon a floating tank installation or a pipe line.

Concrete platforms are largely put together in protected waters, towed to the location in a vertical position and sunken upon arrival. These constructions are not anchored. They are dependent upon their own weight for resisting severe weather. They do, however, have their own storage capacity.

These production platforms and islands are often gigantic constructions. In 1995, a concrete platform was installed in the Troll field on the Norwegian continental plate, measuring 400 meters high and 150 meters long.

For an optimal exploitation of a field, a number of wells must be drilled from the production platform. Using a special drilling technique (drilling diagonally), one can sometimes extract oil or gas from points in the bottom situated several kilometres away from the rig. Even though a fairly large area can be reached using this technique, the average oil or gas field in the North Sea still needs more than one platform.

Safety regulations around the platforms

Mining platforms can form an obstacle for shipping. Likewise, shipping can produce dangers for mining installations. Many platforms in the Dutch North Sea lie in the vicinity of major shipping routes. According to international navigational regulations, the routes themselves must remain empty of installations. The greatest risk involves collisions between platforms and ships which have come adrift. Sea-going ships run adrift around 30 times a year in the North Sea. In the past several decades, offshore platforms had to be evacuated several times due to threatening collisions.

Permanent and temporary safety zones have been established around mining platforms, covering a radius of 500 meters measured from the outer edge of the installation. With the exception of supply ships, it is forbidden to sail or fish in this zone.

From statistics gathered by offshore operators, these safety zones are often violated by all kinds of vessels. Especially fishing vessels like to fish as close as possible to the installations due to the attraction of fish to the area. That often produces dangerous situations.

Based on the Continental Shelf Mining Act, exception areas and restricted areas can be designated on the Dutch section of the North Sea, with the interest of shipping in mind. In these areas, mining activities are forbidden or are tied to special conditions.

'Exhausted' platforms

The first platforms were placed in the North Sea between 1970 and 1980. They have pumped out all of the oil or gas from a field during the past 30 years and have therefore become redundant. Some platforms have been deserted for many years now. Since 1996, platforms in the North Sea have been taken out of production every year. The first platform on the Dutch section of the North Sea was removed in 1996, and several platforms will be dismantled in the future every year. The largest platforms are located in the central and northern section of the North Sea. Some constructions weigh up to 800,000 ton. Removing such an obstacle requires a tremendous amount of lifting capacity, transportation possibilities and safety regulations. This makes the dismantling of drilling platforms very expensive. The classical example is the Brent Spar. Shell wanted to sink this retired oil platform in 1995, but lobbying from Greenpeace prevented this from happening. See the weblinks for more information.

In the southern section of the North Sea, where smaller platforms stand, complete removal of installations and transportation to the mainland is relatively simple. According to the Mining Act, each platform on Dutch North Sea territory which is no longer being used must be completely dismantled, including the substructure. Three quarters of the rigs in the North Sea stand in relatively shallow water no deeper than 55 meters. Just about everywhere in the British and Norwegian section of the North Sea is deeper.

The Groningen authorities are attempting to develop one of its functions by the Ems Harbour for dismantling drilling rigs. In 1997, the Havenschap Delfzijl/Eemshaven gave the green light for shifting the sea dike so that 180 hectares could be created for industrial terrain outside the dike. There is a good chance that the shipping wharves will be built there. The 'mera' (environmental, energy, recycling and waste ) sector, responsible for dismantling the drilling rigs, will also have new opportunities here.

Up till now, the most official directives come from the International Maritime Organization. It has determined that in all cases the top deck with installations and the area directly under sea level must be completely removed. Only those parts that form no obstacle down to 75 meters are allowed under water. And then only as long as there is no new destination for the parts, removal is not technically possible and is extremely costly or causes unacceptable risks.

Parking area for drilling rigs in the Netherlands

In the Management Plan for the Wadden Sea of 1996, a study was announced for finding alternative locations for parking drilling platforms. Two possible locations were found: west of Noorderhaaks in the North Sea and in the harbour of Den Helder. The first location was unacceptable because it was in a military shooting range.

Rare coral on the foot of platforms

In 1999, a number of colonies of the rare coral Lophelia pertusa was discovered in the North Sea, attached to the immersed part of oil platforms. The first colony was discovered when the Brent Spar was removed from the sea. Up till then, the coral was only known from the Atlantic Ocean, where it grows between 150 and 1500 meters deep.

Weblinks

Overview of different kinds of offshore platforms:
http://www.naturalgas.org/naturalgas/extraction_offshore.asp

Greenpeace document on actions around the Brent Spar:
http://www.greenpeace.org/international/about/history/the-brent-spar

Source: de Vleet, Ecomare